Download fast pyrolysis characteristics of sugarcane bagasse hemicellulose

CELLULOSE CHEMISTRY AND TECHNOLOGY
FAST PYROLYSIS CHARACTERISTICS OF SUGARCANE BAGASSE
HEMICELLULOSE
YUNYUN PENG and SHUBIN WU
State Key Laboratory of Pulp and Paper Engineering,
South China University of Technology, Guangzhou, Guangdong, P.R. China
Received August 14, 2010
The pyrolysis characteristics of sugarcane bagasse hemicellulose were investigated using a tubular furnace.
The total gas yield and the releasing behavior of the main gas products were measured by GC analysis. The
liquid products were analyzed by GC-MS and ion chromatography. The chars were investigated by
elemental analysis and proximate analysis. The volatile remaining in the char extracted by isopropanol was
observed by GC-MS analysis. The main gas products from hemicellulose mainly consisted of H2, CO2, CO,
CH4. It was observed that the yield of CO2 and CO was higher at lower temperatures, such as 550 °C, as well
as that of H2 and CH4 at higher temperatures. The liquid products mainly contained furfural, cyclopentene-1
(2-methyl-2-cyclopentene-1, 2-cyclopentene-1, 3-methyl-2-cyclopentene), alcohols (1-butanol), organic
acids (acetic acid, 4-hydroxy-butanoic acid, propanoic acid), aromatic series (phenol, ethylbenzene), besides
some amounts of sugars. The products remaining in the char were mainly alcohols, cyclopentenes and
complex aromatic char.
Keywords: sugarcane bagasse, hemicellulose, pyrolysis, gas products, char, tar
INTRODUCTION
Since biomass materials provide abundant,
inexpensive and renewable resources, their
conversion into synthetic fuels and chemical
feedstock has emerged as an attractive
alternative in the past decade.1 As a first
stage of gasification and liquefaction of
straw and other agricultural materials,
pyrolysis is the key technology of the
thermo-chemical processes. Pyrolysis of
biomass generates three different energy
products in different quantities: coke, gas
and oils. Many studies2-4 showed that fast
pyrolysis gives high oil yields, at a pyrolysis
temperature around 500 °C, gas being the
main product at a temperature over 700 °C.
Consequently, pyrolysis of biomass has
received increased interest since the process
conditions can be optimized to maximize the
production of chars, liquids or gases.
As biomass contains varying amounts of
cellulose, hemicellulose, lignin and a small
amount of extractives, its pyrolysis is a very
complex process. The overall rate of biomass
pyrolysis was considered as a sum of the
individual rates of the three components.5-7
The ratio of the three components in a
biomass, which is closely related to biomass
conversion, differs greatly with the source of
the biomass, so that the study of the
fundamentals of biomass components
pyrolysis is essential. Several studies have
been reported on the pyrolysis of lignin and
cellulose,8-12 but fewer on that of
hemicellulose.
Various agricultural residues, such as corn
fiber, wheat straw and sugarcane bagasse,
contain about 20-40% hemicellulose, the
second most abundant polysaccharide in
nature. Hemicelluloses are heterogeneous
polymers of pentoses (xylose, arabinose),
hexoses (mannose, glucose, galactose), and
uronic acids.13 As hemicelluloses are the
most complex fiber components of straws,
difficult to obtain from natural sources,
numerous
studies
used
xylan
or
4-O-methyl-D-glucurono-D-xylan
(Sigma
from Birchwood) as a substitute of the
hemicellulose component in pyrolysis
processes.5,14,15 Ivan et al.15 used a
thermogravimetric/mass
spectrometric
Cellulose Chem. Technol., 45 (9-10), 605-612 (2011)
YUNYUN PENG and SHUBIN WU
(TG/MS) system to characterize the thermal
reaction
of
4-O-methyl-D-glucurono-D-xylan,
at
atmospheric
pressure,
in
an
inert
environment. Glenn et al.16 synthesized
xylan, to investigate the effects of neutral
inorganic salts on pyrolysis. However, the
real hemicellulose is a matrix of different
polysaccharides, containing glucuronic and
galacturonic acid. It might have different
physical and chemical properties in different
biomass materials, so that improved isolation
procedures and their detailed characterization
are necessary. Shafizadeh et al.17 studied the
pyrolysis
of
D-xylose
and
methyl-β-D-xylopyranoside (selected as
model compounds), as well as that of
4-O-methylglucuroxylan and O-acetyl-4-Omethylglucuroxylan extracted from cotton
wood. The main pyrolysis products were
char, carbon monoxide, carbon dioxide,
water and a tar fraction. Investigations18 have
been carried out on the chemical structure of
hemicellulose isolated from the sugarcane
bagasse and wheat straw, including a
preliminary study on the characteristics of
hemicellulose pyrolysis by TGA and
TG-FTIR. Thermogravimetric analysis had
demonstrated that the main temperature
range required for the thermal decomposition
of hemicellulose (200-315 °C) was
significantly lower than that required for the
decomposition of cellulose (300-415 °C) and
lignin (200-600 °C). The released gas
products from hemicellulose pyrolysis
measured by TG-FTIR were mainly CO2, CO,
H2O and CH4. H2 had no IR absorption and
could not be detected by FTIR.
As part of a continuing study on
hemicellulose pyrolysis, in the present
investigation,
sugarcane
bagasse
hemicellulose was pyrolyzed at temperatures
of 550, 600, 650, 700, 750, 800 and 900 °C,
in a home-made tubular furnace. The main
objective of the present research was to
investigate the effect of pyrolysis
temperature on product distribution, and to
characterize all pyrolysis products by
different analytical techniques, in order to
better understand the fast pyrolysis
characteristics of hemicellulose at higher
temperatures, which will be the basics for the
research on biomass gasification and
liquefaction.
EXPERIMENTAL
Sample preparation
The sugarcane bagasses were provided by the
Jiangmen sugar mill in the Guangdong province,
China. The procedure for the isolation of
hemicellulose was discussed elsewhere.18
Experimental methods
Pyrolysis experiments were carried out under
nitrogen atmosphere, at temperatures of 550, 600,
650, 700, 750, 800 and 850 °C. The pyrolysis
reactor was a tubular furnace equipped with a
quartz tube, tubular furnace and a temperature
control system (Fig. 1), externally heated by an
electrical furnace. Pyrolysis temperature was
measured with an internal thermocouple placed in
the center of the pyrolysis reactor.
Figure 1: Scheme of tubular pyrolyser (1 – N2 gas bottle; 2 – thermocouple with a porcelain boat;
3 – thermocouple; 4 – quartz tube; 5 – tubular furnace; 6 – ice-bath; 7 – gas bottle; 8 – breaker
Before the experiments, 2 g of hemicellulose
(on a dry base) were loaded and the reactor was
purged by nitrogen gas for 10 min, at a flow rate
of 300 mL/min, to remove the air from inside.
The sample was pushed to the center of the
furnace and pyrolysis began when the tubular
furnace was heated to the desired pyrolysis
606
temperature, being pulled out when the sample
reached the same temperature as the furnace. The
gas carried all volatile products from the reactor
into the collection traps. The liquid products were
condensed in three traps containing isopropanol,
by cooling in an ice bath. The non-condensable
volatiles were passed through the last trap,
Sugarcane bagasse
containing salt-saturated fluids, after which the
remaining gases were collected by the
displacement method.
Analysis procedure
The analyses consisted of three stages: gas
product analysis, liquid product analysis and solid
product analysis. The volume of the released gas
was measured by the displacement method and
then converted into the mass. The mass of
residual solid products was measured after they
were cooled to ambient temperature and stored
over dry silica gel until analysis. The weight of
gas was calculated with the volume measured by
the displacement method. The yields of gas and
solid products were calculated based on the initial
mass of the sample, and the yield of liquid
products was calculated according to the
conservation of mass, since the mass of tar was
difficult to measure directly.
Gas product analyses
The gas products were analyzed by gas
chromatography, with a thermal conductivity
detector (GC-20B, Shimadzu, Japan). The C-RID
area method was used to measure the gas
component ratio. Argon gas was used as a carrier
gas, at a flow rate of 77.5 mL/min.
Liquid product analyses
The liquid products from the pyrolysis of
sugarcane bagasse hemicellulose were analyzed
on a gas chromatograph equipped with a mass
selective detector [GC-MSD: 6890N-5975 MSD,
Agilent, America; column: HP-INNOWax,
crosslinked polyethylene glycol, 30 m×0.25 mm,
i.d. 0.25 μm film thickness; oven temperature:
from 40 °C (maintained for 5 min) to 220 °C (rate
5 °C min-1, maintained for 5 min; flow: 1 mL
min-1; split ratio: 20:1].
The aqueous phase of the liquid was first
filtered, then detected by ion chromatography
(ICS-3000, DIONEX, America) for the
identification of sugars. Qualitative analysis of
sugars was conducted with two series-wound
columns of CarborPac PA1 (analytical, 2×250
mm) and CarborPac PA1 (guard, 2×50 mm). The
eluents were 0.001 mol/L sodium hydroxide and
0.05 mol/L sodium acetate. Flow rate of eluent:
0.650 mL/min; injection volume: 10 μL; column
temperature: 30 °C.
Solid product analyses
The solid products were analyzed with an
elemental analyzer (Vario-I element analyzer,
Elementar, France). The analysis of bio-char was
done according to GB2001-91, China, for ash,
volatile matter and fixed carbon analysis.
The bio-char was extracted with isopropanol,
for 24 h, at room temperature. The extract was
analyzed by GC-MS.
RESULTS AND DISCUSSION
Fast pyrolysis of hemicellulose and
product distribution
The distributions of products resulted
from the pyrolysis of sugarcane bagasse
hemicellulose at different temperatures in the
tubular furnace are shown in Figure 2. As
seen,
the
product
distributions
of
hemicellulose changed with increasing
temperature. The gas yield was the highest in
the three phase products. By increasing the
pyrolysis temperature from 550 to 850 °C,
gas yield increased, while char yield
decreased. This result was expected, since it
could be seen from the TG-DTG curves of
hemicellulose that the main mass loss of
hemicellulose occurred between 200 and 315
°C. Hemicellulose, consisting of various
saccharides (xylose, glucose, galactose, etc.),
appears as a random, amorphous structure,
with numerous branches, which are very
easy to remove from the main chain and to
degrade by the released volatiles.19
Composition of gas products
The gas evolving profiles resulted from
hemicellulose pyrolysis in the tubular
furnace are plotted in Figure 3. The gas
products released from the pyrolysis of
hemicellulose were composed of CO2, CH4,
CO, H2, C2H2, C2H4, and C2H6, the major gas
being CO2, as also shown by other
researchers. According to Figure 3, CO2 was
the predominant component of the gas
products at relatively lower temperatures,
decreasing considerably with the temperature
increase. The relative yield of CO first
decreased, from 35.7% at 550 °C to 24.0% at
750 °C, and then increased to 30.12% at 850
°C. The release of CH4 and H2 showed a
contrary pattern to that of CO2.Yang et al.17
suggested that releasing of CO2 was mainly
caused by cracking and reforming of the
functional groups of carboxyl (C=O), while
COOH and CO were mainly released with
the cracking of carbonyl (C–O–C) and
carboxyl (C=O). Possibly, their high yield
was attributed to cracking and abscission of
the C–C and C–O bonds connected to the
main branch of hemicellulose, thus leading
to a high pyrolysis reactivity of
hemicellulose. However, they may have been
released during the second pyrolysis of
volatiles.
607
YUNYUN PENG and SHUBIN WU
Composition of liquid products
The characterization of the liquid
products resulted from hemicellulose
pyrolysis was carried out by GC-MS analysis
and
ion
chromatography.
Ion
chromatography was used for the
identification of sugars generated from the
depolymerization of polysaccharides.
The aqueous phase of the liquid products
was investigated by ion chromatography, the
results showing that xylose and glucose were
found in the liquid products at 550 °C,
together with a small amount of glucose at
600, 650, 700 °C, and no sugar at 750 °C or
above.
Xylose
(76.14%)
was
the
predominant sugar, and glucose (10.72%)
appeared as the second major sugar
constituent in the hemicellulosic fraction.
Additionally, arabinose (9.06%), galactose
(2.37%), uronic acids (1.71%) were observed
as minor constituents.18 Thermogravimetric
analysis demonstrated that the temperature
required for the onset of thermal
decomposition of xylose (218 °C) was higher
than that of D-glucose (230-240 °C),22,23 so
that glucose was more likely to survive the
pyrolysis condition than xylose.
The characterization of the liquid
products was also carried out by GC-MS
analysis. The total ion current programs of
the liquid products at 550 and 750 °C are
shown in Figure 4, and the identification of
the main peaks in the total ion current
program is shown in Table 1.
60
50
H2
CH4
CO
CO2
C2.
50
gas
solid
liquid
45
40
Yield/%
Yield/%
40
35
30
20
30
10
25
0
550
600
650
700
750
800
850
Temperatute/℃
Figure 2: Distribution and yield of products from
hemicellulose pyrolysis
608
550
600
650
700
750
800
850
Temperature/℃
Figure 3: Releasing profiles of the main gas products
from hemicellulose (C2 means C2H2, C2H4, and C2H6)
Sugarcane bagasse
Figure 4: Total ion current (TIC) program of liquid products at different temperatures
The liquid products of hemicellulose
pyrolysis at different temperatures are listed
in Table 1. Most peaks of the chromatograms
were identified by comparing the mass
spectra of the degradation products with the
spectra given in literature.
In all cases, the main pyrolysis products
were classified into the following groups of
compounds: furans [2-furancarboxaldehyde];
cyclopentanes [2-methyl-2-cyclopentene-1,
2-cyclopentene,
3-methyl-2-cyclopentene,
3-methyl-2-cyclopentanedione,
2-hydroxy-3,4-dimethyl-2-cyclopentene];
alcohols [tetrahydro-3-furanol, 1-butanol];
saturated
fatty
acids
[acetic
acid,
4-hydroxy-butanoic acid, propanoic acid];
cyclohexanes [phenol] and aromatic series
[ethylbenzene,
p-xylene,
naphthalene,
indene]. Mainly alcohols, cyclopentanes and
fatty acids were present in the liquid
products at lower pyrolysis temperatures, as
well as aromatic compounds, such as
naphthalene, at higher temperatures.
Acetic acid showed the highest peak at
lower temperatures, such as 550 °C, its
relative content decreasing with the increase
of pyrolysis temperature. Upon pyrolysis,
D-xylose and other sugars with intact
furanose rings could directly form furans.24
2-Furancarboxaldehyde, the typical product
of hemicellulose, was found in the pyrolysis
of xylan and 4-O-methylglucuro-D-xylan.25
Organ and Mackie considered that furan
consumption was ruled by (1, 2)-H transfers,
which resulted in the formation of cyclic
carbene intermediates whose successive
decomposition forms either CO and propyne
or C2H2 and ketene.26 However, some
aromatic compounds were also found in the
tar. At lower temperatures (300-600 °C), the
pyrolysis of polysaccharides typically led to
low
molecular
weight
oxygenated
compounds, including anhydrosugars and
simple aromatic compounds;27 at higher
temperatures (over 800 °C), the pyrolysis of
polysaccharides led to many phenolic and
polynuclear aromatic compounds, observable
at temperatures as low as 550 °C. Of course,
the early stage pyrolysis products can
themselves participate in pyrosynthesis, to
generate larger molecules and more
condensed ring systems. Benzofuran, indene
and other complex aromatic compounds may
be generated by cyclizative condensation of
products at low temperatures.
Table 1
Composition of liquid products from hemicellulose pyrolysis at different temperatures
No
1
2
3
4
5
R.T.,
min
4.41
4.52
4.521
4.61
5.25
Compounds
ethylbenzene
p-xylene
1-butanol
p-xylene
p-xylene
550 °C
10.32
2.06
4.88
3.03
600 °C 650 °C
7.12
9.06
1.95
3.87
4.98
2.40
3.13
Area, %
700 °C 750 °C 800 °C 850 °C
8.69
1.78
1.82
2.11
1.95
6.13
5.21
1.34
2.4
2.94
0.73
1.16
609
YUNYUN PENG and SHUBIN WU
6
7
8
6.42
7.20
8.17
9
8.36
10
11
12
13
14
8.47
9.86
10.08
10.81
10.43
15
11.02
16
17
18
11.36
12.98
14.76
19
15.52
20
16.08
21
22
23
24
25
16.48
17.03
18.69
18.75
19.92
styrene
1-hydroxy-2-propanone
2-cyclopenten-1-one
2-methyl-2-cyclopenten-1-on
e
tetrahydro-3-furanol
acetic acid
2-furancarboxaldehyde
benzofuran
indene
3-methyl-2-cyclopenten-1-on
e
propanoic acid
4-hydroxy-butanoic acid
naphthalene
2-hydroxy-3,4-dimethyl-2cyclopenten-1-one
3-methyl-2-cyclopentanedion
e
2-methyl-naphthalene
1-methyl-naphthalene
2-methyl-phenol
phenol
3-methyl-phenol
3.60
1.19
0.78
3.53
1.58
1.71
2.63
-
3.21
1.81
1.96
7.16
-
6.70
-
5.17
-
1.75
1.46
-
1.02
-
-
-
1.68
41.18
3.56
-
1.49
52.55
3.21
-
1.28
36.24
3.19
-
24.31
1.55
1.83
2.03
0.18
1.42
8.55
1.94
9.05
5.91
0.56
1.66
5.32
0.96
0.95
-
-
-
-
-
4.20
1.22
-
5.55
1.45
-
4.86
1.04
-
3.24
1.02
-
43.76
42.63
0.78
28.58
0.49
-
-
-
-
-
-
1.04
0.64
-
-
-
-
-
4.06
4.06
1.45
7.85
3.00
1.46
1.04
8.76
3.14
3.29
2.22
13.39
4.40
4.50
2.63
8.07
-
4.58
2.82
8.74
-
5.35
3.56
13.99
6.15
Composition of solid products
Some physico-chemical properties of
bio-chars resulting from the pyrolysis of
hemicellulose at 550, 600, 750 and 850 °C
are shown in Table 2. As expected, the
amount of volatile matter in bio-char was the
highest at 550 °C, besides other pyrolysis
temperatures. By increasing temperature
from 550 to 850 °C, the amount of fixed
carbon of bio-char increased from 16.52 to
22.98 wt%, while the ash content of
bio-chars increased only slightly. The
ultimate analyses of bio-chars showed that
the carbon amounts were first increased and
then decreased with increasing the pyrolysis
temperature. The molar ratio of C/H and C/O
decreased when the pyrolysis temperatures
were over 650 °C, which was caused by the
secondary cracking of primary volatiles. The
condensation and polymerization reaction of
the primary volatile took place at higher
temperatures, leading to the generation of
more thermostable compounds, such as
benzene and naphthalene.
During pyrolysis, volatile components
were released and absorbed by the
condensation trips, however, some residual
organics still remained in the solid char.
Table 2
Properties of bio-chars from pyrolysis of hemicellulose at different temperatures
Pyrolysis temperature
Proximate analysis
Volatile matter
Fixed carbon
Ash
Ultimate analysis (dry wt%)
C
H
O
N
S
C/H molar ratio
C/O molar ratio
610
550 °C
600 °C
750 °C
850 °C
37.04
16.52
18.50
30.23
18.01
19.44
28.33
20.32
21.01
26.46
22.98
22.96
47.80
3.76
48.27
0.0055
0.0895
12.72
0.99
50.90
3.49
45.55
0.0040
0.0535
14.57
1.12
51.47
2.25
46.20
0.0090
0.0731
22.87
1.11
40.48
2.35
57.10
0.0085
0.0691
17.24
0.71
Sugarcane bagasse
Table 3
Chemical composition of the iso-propanol soluble fraction in char hemicellulose at 550 °C
No
R.T.,
min
Compounds
1
3.184
1-methylethyl ester
formic acid
2
3
3.221
3.500
methyl isobutyl
ketone
Structure
No
R.T.,
min
Compounds
8
4.650
2-hexanol
9
4.961
tetrahydro-6,6-dimethyl2H-pyran-2-one
Structure
O
OH
O
O
OH
1-propanol
10
5.160
O
O
2-methyl-3-pentanol
OH
4
3.597
toluene
5
4.134
2,3-dimethyl-2butanol
6
4.258
2-methyl-2-pentanol
OH
OH
11
10.626
2-methyl2-clyclopentane-one
12
10.965
furfural
13
11.249
OH
7
4.488
3-methyl-3-pentanol
The chemical composition of the
isopropanol-soluble fraction in char was
detected by GC-MS analysis. The chemical
structure and molecular weights of these
compounds are listed in Table 3.
As
shown
in
Table
3,
the
isopropanol-extracted fractions in the char at
550 °C included alcohols, anhydrosugars,
acids and furfural, besides complex aromatic
compounds. Some kinds of alcohols were
also detected, because of the isopropanol
used as a solvent. However, the complex
aromatic compounds were the predominant
components remaining in the char of higher
temperatures, and only a few low molecular
weights were detected. Indeed, polynuclear
aromatic compounds were the kinetically and
thermodynamically preferred final products
of
high
temperature
pyrolysis
of
carbohydrates.24
CONCLUSIONS
The products distribution and yields of
hemicellulose pyrolysis in the tubular
furnace
were
studied
at
different
temperatures. The yield of gas was the
highest of the three-phase products,
decreasing gradually with the increase of
pyrolysis temperature, while the char yield
was contrary to that of the gas products. The
incondensable gas products were CO2, CH4,
CO, H2, and some light hydrocarbon (C2H2,
C2H4, and C2H6). CO2 was the predominant
14
21.382
1,6 ;3,4-dianhydro-2-deoxyβ-d-lyxo-hexopyranose
4-(1-methylethoxy)
-1-butanol
O
O
O
O
O
O
O
OH
component of the gas products at relatively
lower temperatures, decreasing considerably
with the increase of temperatures. The
relative yield of CO first decreased and then
slightly increased. The release of CH4 and H2
showed a contrary pattern to that of CO2. CO
was not released only from primary pyrolysis,
but also from the second pyrolysis of the
volatile products, whereas CH4 and H2
resulted mainly from the latter process. The
liquid products included sugars (glucose,
xylose), furfural, cyclopentene derivatives,
alcohols, saturated fatty acids (acetic acid,
4-hydroxy-butanoic acid, propanoic acid),
phenol, phenol methyl-/dimethyl- derivatives
and other aromatic compounds. Aromatic
compounds, such as naphthalene, increased
obviously with increasing temperature. The
ultimate and proximate analysis of char
showed that the amount of volatile matter
decreased, while that of fixed carbon and ash
increased by increasing temperature from
550 to 850 °C. The isopropanol-extracted
fractions of volatile compounds remaining in
the char were mainly alcohols, cyclopentene,
and other complex compounds.
According to the analysis of three-phase
products resulting from hemicellulose
pyrolysis, dehydration and decomposition of
the side chain of hemicellulose took place in
the beginning of pyrolysis, after which
cleavage of the glycosidic, carbon-carbon,
carbon-hydrogen and carbon-oxygen bonds
611
YUNYUN PENG and SHUBIN WU
of the sugar units, depolymerization,
decarboxylation
and
decarbonylation
reactions took place in the main pyrolysis
stage, while the C-C, C=C, C-O bonds, as
well as the carbonyl and carboxyl groups
were reformed by the free radical reaction
and the energy of high temperature. Low
molecular weight aldehydes, alcohols, furan
and polynuclear aromatic compounds were
the observable end-products.
ACKNOWLEDGEMENTS:
The
investigation was supported by a grant from
the Major State Basic Research Development
Program of China (973 Program) No.
2007CB210201, and the National High
Technology Research and Development
Program of China (863 Program) No.
2007AA05Z456.
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